The invention disclosed herein is a matched filter for use in connection with temporal subtraction of X-ray images.
Digital fluoroscopy apparatus and methods are used for visualizing the flow of an X-ray contrast medium through blood vessels. One digital fluoroscopy modality involves projecting an X-ray beam through a body, converting the resulting X-ray image to an optical image with an image intensifier, converting the optical image to analog video signals with a video camera and then digitizing the video signals to form a matrix of digital values that correspond in magnitude to the intensity of the picture elements (pixels) that compose the image. In temporal imaging, an image of a region of the anatomy that contains the blood vessels of interest is obtained before an intravenously injected X-ray opaque medium reaches the vessels. This image is typically stored as a mask image. When the X-ray contrast medium begins to flow through the vessels, a series of live images are obtained. The mask image is then subtracted from the successive live images to produce a sequence of difference images. The object of subtraction is to cancel all image content such as bone and soft tissue which is unchanged in the mask and live images and let the image of the contrast medium containing blood vessels remain for display. As is known, the pre-contrast mask images and post-contrast images will always have some noise content that is introduced by the X-ray system and the electronic components that are used to generate and process the signals that represent the image.
One method that has been used to reduce the effect of noise is to integrate several successive images on the assumption that, since noise is a random phenomena, it will cancel out. Integration over a long period of time is not fully satisfactory, however, because it introduces a greater probability that the body being examined will have moved during the integration interval in which case motion artifacts become evident in the visible image.
Recursive filtering has been proposed for reducing the effect of noise in temporally subtracted X-ray images, that is, in the difference image that results from subtracting a mask image obtained at one time from a live contrast medium exhibiting image obtained shortly thereafter. Recursive filtering in temporal subtraction systems was recently described in several articles: Kruger, R. A. "A Method for Time Domain Filtering Using Computerized Fluoroscopy": Medical Physics, Vol. 8, No. 4, July/August 1981, pp. 465-469; Kruger, R. et al, "Time Domain Filtering Using Computerized Fluoroscopy--Intravenous Angiography Applications", SPIE Vol. 314 Digital Radiography (1981), pp. 319-326; Gould, R. G. et al "Investigation of a Video Frame Averaging Digital Subtraction System", SPIE Vol. 314, pp. 184-190 (1981); and, Gould, R. G. et al "A Digital Subtraction System With Tandem Video Processing Units," SPIE Vol. 273, pp. 125-132 (1981). The apparatus and method described in these articles assumes prior knowledge of the manner in which the concentration of contrast medium in the blood vessels of interest varies with time. Generally speaking, a plot of concentration versus time results in a curve that bears a rough resemblance to a Gaussian distribution curve but, more specifically is usually modeled by gamma variate wherein there is a relatively low concentration of contrast medium when the medium first reaches the blood vessels of interest and then it reaches a peak concentration followed by a decline until the vessel is again occupied by blood that does not contain any contrast medium. By way of example, some contrast medium may be present over an interval of 15 or more seconds whereas, the time of interest existing between the two half-maximum points on the plot may be a 5-10 second interval. Two recursive filter channels are used in the X-ray image subtraction system described in the first two cited articles. Each effectively converts the contrast medium or bolus flow characteristics from the time domain to the frequency domain and the ultimate result of cooperative action between the two filters is to effectuate a band pass filter in whose output signal noise and unchanged pre-contrast and post-contrast structures are cancelled out and an image of the contrast medium containing vessels remains.
The digitized video signals for each pre-contrast and post-contrast image in a sequence are input to the recursive filter channels simultaneously. Each channel has a full-frame memory in which a fractional amplitude portion of the sum of all previous or earlier image frames are added to a fractional amplitude portion of the live or present video signal constituting a frame such that the relative importance of a signal n frames previous is determined by the value of a coefficient "K". For example, if "K" were equal to 0.5 and (1-K) were equal to 0.5, the output signal from a memory would consist of 1/2 of the present signal, 1/4 of the next earlier frame signal, 1/8 of the next earlier frame, 1/16 of the next frame behind that and so on such that the signal 7 or 8 frames preceding the present of live signal has little weight. When a multiplicity of such identical signals are summed, the result is a signal identical to any one of the summed signals and of the same magnitude as the unattenuated incoming live video signal because the sum of "K" and (1-K) is always unity. However, when random noise present in the video signal, which is independent from frame to frame, is summed, it tends to be cancelled or in any case not be reinforced as is the periodic video signal. It can be demonstrated that the improvement in the signal-to-noise ratio with this scheme is equal to 10 log (2-K)/K db.
Thus, if the value of K were 0.5, the value of the fraction would be 3 and the logarithm would indicate a 4.7 db signal-to-noise ratio improvement. Similarly, if K were smaller, such as 0.3, the signal-to-noise (SNR) improvement would be about 7.53 db.
In one recursive filter channel, the video signal is fed through an attenuator that inputs the value of KxL (live video) to a summer. The summer output is an input to a full-frame memory. The output of the full-frame memory is another input to the summer and in this loop, the stored or accumulated video signal is multiplied by (1-K). The other recursive filter channel functions in the same way except that it uses a different coefficient K. The contents of the memories for the common frame in each of the recursive filter channels are then subtracted to produce a net difference image digital frame format. This is reconverted to analog video signals for display on a television monitor.
The imaginative concept of using two recursive filters to achieve a pass band from which noise is excluded has resulted in a significant improvement in SNR over previously known noise reduction schemes for X-ray image subtraction. Applicants, however, recognized that in the described system, noise reduction is to some extent achieved at the expense of useful signal reduction where useful signal is that which represents the X-ray contrast medium. In other words, a two channel recursive filtering system does not use the image representative signals with maximum efficiency. Useful signal is cancelled out by reason of two different values of K being used in the respective filter channels. In reality, this means that the two channels have different time constants. Thus, for a frame obtained at any time along the bolus interval or, in other words, when contrast medium is present in the vessel, the filter with the fastest time constant will have practically no remnant of frames that were taken far back in time whereas the slow time constant filter may still have a significant amount of signal carried over from frames obtained in the more distant past. Moreover, the slow time constant filter, which should theoretically have the data representative of a pre-contrast mask, actually contains some signal that was developed after contrast medium started to pass across the X-ray beam. Thus, when the two images are subtracted some contrast medium signal is cancelled and total contrast medium signal is reduced undesirably.